ORTHOPYROXENE

General Formula:
(Mg,Fe)2Si2O6

Sample: W-3

System:
Orthorhombic

Orthopyroxene
The similar appearence in plane light of orthopyroxene and clinopyroxene are highlighted in this image. Note that both grains lack any clear colour, although coloured and pleochroic varieties are common.
Field of view = 4 mm, plane light

Orthopyroxene
The low interference colours characteristic of orthopyroxene compared to clinopyroxene are evident in this image.
Field of view = 4 mm, crossed polars

Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

the effects of the substituion of Fe for Mg in orthopyroxene on the optical properties is evident in examining Figure 13.4 in Nesse.

Alteration

alters to serpentine, talc or fine grained amphibole

Occurrence

Mg-rich opx is common in mafic intrusive rocks (gabbro, norite, etc.). Fe-rich opx is found in more siliceous igneous rocks (diorite, syenite etc.).
opx is common in high grade regional metamorphic rocks